Ultrahigh vacuum magnetron ionization gauge with ferromagnetic electrodes

ABSTRACT

A magnetron ionization gauge having the capability of measuring ultrahigh vacuum of the order of 2 X 10 16 torr with high sensitivity and conversion constant is formed with a hollow cylindrical anode and an electron-containment ferromagnetic plate at either end of the anode. An ion-collector is centrally located within a cylindrical anode and extends the entire length thereof. An electron-emitting cathode is located off center between the ion-collector and the anode wall and parallel to the longitudinal axis. An ion-collector shield extends the entire length of the ion-collector and is interposed between the ion-collector and the electron-emitting filament. The device is biased well beyond cutoff to provide restricted curvilinear paths for ionizing the electrons circulating around inside the anode.

United States Patent [72] Inventor Louis J. Favreau Elnora, N.Y. [21]Appl. No. 826,815 [22] Filed May 22, 1969 [45] Patented June 1,1971 [73]Assignee General Electric Company [54] ULTRAHIGH VACUUM MAGNETRONIONIZATION GAUGE WITH FERROMAGNETIC ELECTRODES 9 Claims, 1 Drawing Fig.

[52] U.S.Cl 315/108, 313/7, 313/156, 313/157, 324/33 [51] hit. Cl 1101i7/16, HOlj 17/22, GOln 27/62 [50] Field ofSearch 324/33;315/108,111;313/7,153,l56,157,162,231; 230/69 [56] References CitedUNITED STATES PATENTS 3,051,868 8/1962 Redhead 315/108 3,172,597 3/1965Guyot 3l3/7X TO END PLATES AND SHIELD WA lee 3,387,175 6/1968 Lloyd etal 315/108 3,495,127 2/1970 Lafferty 315/108 3,505,554 4/1970 Vekshinskyeta 313/157 ABSTRACT: A magnetron ionization gauge having the capabilityof measuring ultrahigh vacuum of the order of 2 X 10 ton with highsensitivity and conversion constant is formed with a hollow cylindricalanode and an electron-containment ferromagnetic plate at either end ofthe anode. An ion-collector is centrally located within a cylindricalanode and extends the entire length thereof. An electron-emittingcathode is located off center between the ion-collector and the anodewall and parallel to the longitudinal axis. An ion-collector shieldextends the entire length of the ion-collector and is interposed betweenthe ion-collector and the electron-emitting filament. The device isbiased well beyond cutoff to provide restricted curvilinear paths forionizing the electrons circulating around inside the anode.

PATENTEUJUN 1 197+ TO END PLATES AND SHIELD VIA LEAD 42 IN VE NTOR:

LOU/8 .1. F4 VRE u y HIS ATTORNEY ULTRAIIIGI-I VACUUM MAGNIJTRONIDNIZATIQN GAUGE WITH FERROMAGNE'IIC ELECTRODES The present inventionpertains to improvements in magnetron ionization gauges and, moreparticularly, to such devices which are capable of operating in theultrahigh vacuum region and which are adapted to operate with a veryhigh conversion factor to record extremely low pressures.

As is known in the art, ionization gauges are utilized to measure lowgas pressures by measuring the number of positive ions created by thecollision of electrons with gas molecules present within ananode-cathode space. The positive ions are attracted to a negativelybiased ion-collector electrode and the current created thereby is ameasure of the gas pressure within the device.

US. Pat. application Ser. No. 708,124, entitled Ultra-High VacuumMagnetron Ionization Gauge with Ion-Collector Shield," filed Feb. 26,1968 by J. M. Lafferty and of common assignee, discloses a magnetronionization gauge with improved operation and the ability to measurepressures as low as torr. Devices in accord with that inventiongenerally comprise a cylindrical anode, a pair of electron-containmentend members, a longitudinally centered semicylindrical ioncollectorelectrode, an offcentered longitudinal electronemitting filament betweenthe ion-collector electrode and the inner wall of the anode electrodeand a shield electrode member interposed along the axis of the anodeelectrode between the ion-collector and the electron-emitting filament.Improved sensitivity and a higher conversion factor is achieved byoperating the device well beyond cutoff and by applying a more positivepotential to the shield electrode than that which is applied to theion-collector to cause high energy electrons in the circulating spacecharge to be intercepted by the shield electrode, thereby reducingelectron currents which would otherwise counteract the currents due tothe attraction of positive ions and tendency to render the gaugeinaccurate.

While the invention described in the aforementioned patent applicationprovides a great improvement over prior art ionization gauges, thedemands of an ever-expanding technology require the development ofgauges with still further improved low pressure measuring abilities.

It is, therefore, an object of the present invention to provide amagnetron ionization gauge with increasedv sensitivity, reduced X-rayphotoemission, and a higher conversion factor so that there are highermeasurable currents per unit of pres.- sure measured.

Another object of the invention is to provide a magnetron ionizationgauge having a more uniform magnetic field along the entire length ofthe ion-collector electrode.

Briefly stated, the present invention provides a magnetron ionizationgauge including a cylindrical anode, a pair of ferromagneticelectron-containment end members therefor, a longitudinally centered,semicylindrical ion-collector electrode along the length dimensionthereof, an offcentered, longitudinal electron-emitting filament betweensaid ion-collector electrode and the inner wall of the anode electrode,and a shield electrode member interposed along the axis of the anodeelectrode between the ion-collector electrode and the electron-emittingfilament. In operation, the gauge is biased with voltages and alongitudinal magnetic field such as to cause the device to operate wellbeyond cutoff, further minimizing the possibility of X-rayphotoemission. Additionally, the ferromagnetic electron-containment endmembers provide a more uniform magnetic field along the axis of theion-collector electrode thereby increasing the measurable current perunit of pressure measured. Additionally, by the introduction offerromagnetic electron-containment end members, there is an attendantincrease in the magnetic flux density which permits reduction in themagnitude of the longitudinal magnetic field required for the device.

The novel features believed characteristic of the present invention areset forth in the appended claims. The invention itself, together withfurther objects and advantages thereof, may best be understood withreference to the following detailed description, taken in connectionwith the appended drawing.

The FIGURE is a vertical cross-sectional view of a magnetron ionizationgauge constructed in accord with the present invention.

In the FIGURE there is illustrated a magnetron ionization gauge,represented generally as 10, including an hermetically sealable envelope11, comprising a flanged, cylindrical member 12, closed at one end by anapertured metal end plate 13 and at the other end by an aperturedannular end plate 14 into which a tubulation 48, which connects gauge 10to a system to be monitored, is fitted. Within envelope 11, theoperative elements of the gauge include a hollow cylindrical anodemember 15, a pair of electron-containment members or plates 16 and H7juxtaposed at either end of, and not in contact with, anode 15, athermionically emitting filament 18,0ffset from the longitudinal axis ofanode 15, a semicylindrical ion-collector member 19 extending the entirelength of the anode electrode and symmetrically located about thelongitudinal axis thereof, and a shield electrode member 20, also ofsemicylindrical configuration, juxtaposed between ion-collector l9 andthermionic filament l3 and concentric with the longitudinal axis of theanode member 15.

Electron containment-end plates 16 and 17 are preferably circular discsof ferromagnetic material such as pure iron; however, it is understoodthat other configurations or materials are contemplated, all within thescope of this invention. For example, other ferromagnetic materials suchas cobalt, nickel, gadolinium, or alloys of any of these materialsinclud ing iron could likewise be used. It is understood, of course,that the foregoing list of materials is merely for purposes ofillustration and not by way of limitation. Obviously, otherferromagnetic materials could likewise be used. The end plates 16 and 17are mechanically and electrically secured to shield electrode 20. Anodeelectrode 15 is supported upon a pair of anode support members 2i and22. Support member 21 is connected to a lead member 23, which passesthrough a leadin bushing 24 and is hermetically sealed therethrough tothe exterior of the device. Support member 22 is connected to a leadmember 29 which passes through a lead-in bushing 30.

Lead-in bushing 24 comprises a longitudinally apertured ceramic member25, having a bore therein sufficient to accommodate lead member 23without making contact thereto. An inner, metallic flanged member 26 ishermetically sealed through an aperture in annular end plate M andsurrounds insulator 25 and is sealed thereto by conventionalmetalceramic technique at flanged end 27 thereof. At the other end ofinsulator 25, an end cap member 28 is sealed in conventionalceramic-to-metal hermetic seal to insulator 25 and is hermeticallysealed as by brazing, or otherwise, to lead member 23 as it exits fromthe bushing. Lead-in bushing 30 is similarly constructed.

loncollector electrode 19 is supported from a lead member 35 whichpasses through a lead-in bushing 36 including insulator 37, innerflanged seal 38, hen'netically sealed to insulator 37 at flanged end 39,and exterior flanged seal 40, which is in hermetic seal with the outerend of insulator 37 and is brazed or otherwise hermetically sealed tolead member 35 as it passes through the bushing. End members 16 and 17are interconnected with shield electrode 20 and are supported on a leadsupport member 41 which is connected to a lead member 42, which issealed through end member M- by a bushing 43 and by anotherdiametrically opposed support, not shown. Filament I8 is electricallyand mechanically connected to the upper anode end plate member l6 andpasses through an aperture in the lower end plate member 17 and issupported upon a support member 44. Support member 44 is connected to alead member 45 which is hermetically sealed through the envelope endmember 14 through a bushing 46.

Each of the bushings 24, 3t), 36, 43, and 46 are so constructed that thelead wire passing therethrough makes no contact with the insulatingmember. Similarly, the inner portions of the bushing seals are soconstructed that no contact is made with the inner portion of theinsulator member and hermetic seal is made thereto only at the outerflanged end. Both of these measures greatly increase the insulatingcharacteristic of the bushing, in that the bushing provides a longinsulating surface path between the lead passing therethrough and themetallic member of the bushing exposed within the device, to preventcovering of the surface of the insulator and the short circuiting of theelectrode leads passing therethrough.

The materials from which the ionization gauge is constructed areconventional; for example, they may be made of molybdenum, stainlesssteel or any other suitable nonmagnetic refractory metal, except endmembers 16 and 17 which, as previously described, are made offerromagnetic material. The filament 18 is preferably a lanthanum boridecoated rhcnium substrate such as is disclosed in US. Pat. No. 3,312,856.

Dimensionally, anode electrode may have an inner diameter ofapproximately one inch and a longitudinal length of approximately oneand one-eighth inches. The circle described by the exteriorconfiguration of the semicylindrical ion-collector 19 and shieldelectrode 20 may conveniently have an outside diameter of approximately0.25 inch. Thermionic cathode 18 may conveniently be a 0.008 inchrhenium wire, lanthanum boride coated, and is geometrically located atapproximately half the distance between shield electrode member 20 andthe inner surface of anode electrode 15. Thus, for example, the centerof the filament, in the dimensions mentioned above, would bethree-sixteenths inch from either of shield electrode 20 and anodeelectrode 15. The electroncontainment end plates 16 and 17 mayconveniently have a diameter of one inch and a thickness ofone-sixteenth inch for the embodiment illustrated. However, the diameterof the end plates 16 and 17 is primarily dependent on the diameter ofthe anode electrode. The thickness of the end plates while not critical,may vary slightly with the type of material used. For example, slightlythicker end plates may be used if cobalt or nickel and their alloys areused instead of pure iron.

Voltage means are provided by a voltage supply, generally indicated as50 and comprising, for example, a battery 51 and a voltage dividingresistor 52. Anode electrode 15 may conveniently be operated at apotential of approximately 300 volts positive, with end plate members 16and 17 and shield electrode 20 operated at a potential of 0 volts, andion-collector electrode 19 at a potential of approximately 100 voltsnegative. The upper end of filament 18 is connected to upper end platemember 16 at 0 voltage. The lower end of the filament is connected tobattery 51 through a lead 45 causing a voltage of approximately 3 voltsto be impressed on filament 18. A strong longitudinal magnetic field,represented by the arrow H, of approximately 600 oersteds, for example,is applied within the anode to cause magnetron action. This field,generated by either an electromagnetic or a permanent magnetappropriately positioned around the ionization gauge, together with theapplied electric field is approximately four times that necessary tobias the device to cutoff. An ion-current measuring meter 53 isconnected in the ion-collector circuit through a lead member 35 and isused as an indicator of collector ion current and, hence, is a measureofgas pressure.

in operation, filament 18 is operated at a very low temperature ofapproximately 650 to 800 C. This is possible because of the lanthanumboride cathode which is an excellent electron emitter at relatively lowtemperatures. The electrons emitted from the electrode 18 initially tendto be accelerated to the anode 15; however, due to the crossed electricand magnetic fields existing within the interaction space therein, theelectrons execute extended curvilinear paths, generally cycloidal orhelicoidal in shape and circulate about the filament in a spiral pathcreating a negative space charge. Due to the very high magnetic field,electrons are precluded from approaching too close to the anode.

The device described in the aforementioned patent application by JamesM. Lafferty, adjusted the magnetic field and cathode-anode voltage sothat the device operated far past cutoff to reduce the probability ofelectrons impinging upon the anode. In addition to operating beyondcutoff, the instant invention utilizes the ferromagnetic end platemembers 16 and 17 to create a more uniform magnetic field along thelength of the ion-collector electrode 19 thereby permitting operation ateven lower magnetic field intensities. Whereas the magnetic field alongthe longitudinal axis of the ion-collector 19 tends to have a lowervalue at the vicinity of the ends thereof when the end members 16 and 17are not ferromagnetic, by the use of ferromagnetic end members 16 and 17the present invention insures a substantially constant magnetic fieldalong the entire length of the ion-collector 19. Several importantadvantages are thereby obtained.

One important advantage gained by the use of ferromagnetic end members16 and 17 is that the cutoff current to the anode is greatly decreased.Performance tests for an ionization gauge constructed in accord with theinstant invention have demonstrated a decrease in incidence of electronbombardment of the anode and hence a reduction in cutoff current ofapproximately 25 percent when compared with the device described in theaforementioned Lafferty patent application. As a result, there is adecrease in X-ray photoemission at the ion-collector, thereby greatlydecreasing erroneous indications of positive ion currents.

A second advantage which results directly from the decrease in cutoffcurrent to the anode, is a substantial increase in the sensitivity ofthe gauge. Whereas the gauge described in the aforementioned Laffertypatent application is capable of measuring pressures of the order of 10'torr with a conversion constant of approximately 0.5 ampere per torr,gauges in accord with the present invention are capable of measuringpressures of the order of 2.0 l0" torr with a conversion constant ofapproximately 0.8 ampere per torr. Accordingly, in addition to theincreased sensitivity, the ioniza tion gauge of the present inventionalso has an improved conversion constant. This results directly from theuse of the ferromagnetic end members which provide a more uniformmagnetic field along the longitudinal axis of the ioncollector, therebyincreasing the efficiency of collecting ions at the ioncollectorelectrode 19.

In accord with another feature of the present invention, ionizationgauges embodying the invention are more stable than prior art devices.It has been found that the electrons are used so efficiently in devicesof the instant invention that an increase or a decrease in electronsoccasioned by a change in emission from the filament does not affect theion current appreeiably. Accordingly, the instant invention hasconverted a previously critical parameter of ionization gauges to onewhich no longer requires critical control.

In accord with another feature of the present invention, it has beenfound that the relative position of a permanent magnet or electromagnetused to create the magnetic field surrounding the ionization gauge is nolonger as critical as in prior art devices.

In accord with still another feature of the present invention, it hasbeen found that the ferromagnetic end plates may be used to achievespecial magnetic field variations. This is readily accomplished byvarying the thickness or shape of the ferromagnetic end plates so as tochange the magnetic flux density along the axis of the ion-collector.This feature is particularly useful where it may be necessary tocompensate for the effects of extraneous magnetic fields which may havea tendency to alter the desired magnetic field along the length of theion-collector.

From the foregoing, it may be appreciated that there is described animproved magnetron ionization gauge having the capability of measuringion pressures down to as low as 2X10 torr and having high conversionconstants measured in amperes per torr of gas measured. This is achievedby utilizing a longitudinal ion-collector along the axis of a hollowcylindrical anode electrode with ferromagnetic electron-containment endmembers to provide a more uniform magnetic field along the entire lengthof the ion-collector electrode. ln further accord with the invention,there is provided a shield electrode for the ion-collector at apotential intermediate that of the ion-collector and anode to remove,from the circulating electron space charge, abnormally high energyelectrons to preclude the bombardment of the ion-collector electrodethereby and the consequent diminution of the apparent positive ioncurrent. Additionally, there is provided an offcenter, low temperaturefilament, shielded from the ion-collector to preclude erroneous ioncurrents and adapted to supply the necessary electrons to provide forionizing electrons within the device.

While the invention has been set forth herein with respect to certainparticular embodiments and examples, many modifications and changes willreadily occur to those skilled in the art.

What I claim as new and desire to secure by Letters Patent of the U.S.is:

1. An ionization gauge adapted to measure ultrahigh vacua at highsensitivity and stability and comprising:

an hermetically sealable envelope adapted to be attached to a vacuumsystem;

a hollow cylindrical anode electrode within said envelope;

a pair of ferromagnetic end members disposed in spaced relationship toone another at opposite ends of and insulated from said anode electrode;

an ion-collector electrode disposed adjacent the longitudinal axis ofsaid anode electrode extending substantially the entire length thereofand parallel thereto;

a thermionic filament disposed within said anode electrode spacesubstantially parallel to the longitudinal axis thereof, extendingsubstantially the entire length thereof, and offset from thelongitudinal axis of said anode electrode;

a shield electrode disposed parallel to the longitudinal axis of saidanode electrode, on the opposite side of said axis from saidion-collector electrode, interposed between said ion-collector electrodeand said thermionic filament, and extending substantially the entirelength of said anode electrode;

bias means for applying a positive potential to said anode electrode, afirst negative potential to said ion-collector electrode, and a secondsubstantially less negative potential to said shield electrode;

means for providing a longitudinal magnetic field to said devicesufficient, in connection with the radial electric field existingbetween said ion-collector electrode and said anode electrode, to biassaid device well beyond cutofi' so that the magnetic field applied ismuch greater than that just necessary to prevent electrons from reachingsaid anode electrode; and

said applied potentials being of a magnitude as to cause electronswithin said anode space to describe elongated curvilinear paths aboutsaid ion collector and for electrons achieving abnormally high energiesto be attracted to said shield electrode and removed from said space.

2. The gauge of claim 1 wherein said ferromagnetic end members are madeof iron.

3. The gauge of claim 1 wherein said ferromagnetic end members arealloys of iron.

4. The gauge of claim 1 wherein said ferromagnetic end members aremechanically and electrically supported by said shield electrode.

5. The gauge of claim 2 wherein said ferromagnetic end members aremechanically and electrically supported by said shield electrode.

6. In combination with a magnetron ionization gauge of the type having alongitudinally extending ion-collector electrode centrally positionedwithin a cylindrical anode and an electron emitting cathode locatedoffcenter between said ion-collector and said anode and a shieldelectrode interposed along the axis of the anode electrode between theion-collector electrode and the electron-emitting electrode, theimprovement comprising:

a ferromagnetic electron-containment member adjacent one end of saidanode electrode. I 7. The combination recited in claim ll wherein saidferromagnetic electron-containment member is mechanically andelectrically secured to said shield electrode.

8. The combination recited in claim ll comprising a second ferromagneticelectron-containment member adjacent the other end of said anodeelectrode.

9. The combination recited in claim 5 wherein said secondelectron-containment member is mechanically and electrically secured tosaid shield electrode.

1. An ionization gauge adapted to measure ultrahigh vacua at highsensitivity and stability and comprising: an hermetically sealableenvelope adapted to be attached to a vacuum system; a hollow cylindricalanode electrode within said envelope; a pair of ferromagnetic endmembers disposed in spaced relationship to one another at opposite endsof and insulated from said anode electrode; an ion-collector electrodedisposed adjacent the longitudinal axis of said anode electrodeextending substantially the entire length thereof and parallel thereto;a thermionic filament disposed within said anode electrode spacesubstantially parallel to the longitudinal axis thereof, extendingsubstantially the entire length thereof, and offset from thelongitudinal axis of said anode electrode; a shield electrode disposedparallel to the longitudinal axis of said anode electrode, on theopposite side of said axis from said ion-collector electrode, interposedbetween said ioncollector electrode and said thermionic filament, andextending substantially the entire length of said anode electrode; biasmeans for applying a positive potential to said anode electrode, a firstnegative potential to said ion-collector electrode, and a secondsubstantially less negative potential to said shield electrode; meansfor providing a longitudinal magnetic field to said device sufficient,in connection with the radial electric field existing between saidion-collector electrode and said anode electrode, to bias said devicewell beyond cutoff so that the magnetic field applied is much greaterthan that just necessary to prevent electrons from reaching said anodeelectrode; and said applied potentials being of a magnitude as to causeelectrons within said anode space to describe elongated curvilinearpaths about said ion collector and for electrons achieving abnormallyhigh energies to be attracted to said shield electrode and removed fromsaid space.
 2. The gauge of claim 1 wherein said ferromagnetic endmembers are made of iron.
 3. The gauge of claim 1 wherein saidferromagnetic end members are alloys of iron.
 4. The gauge of claim 1wherein said ferromagnetic end members are mechanically and electricallysupported by said shield electrode.
 5. The gauge of claim 2 wherein saidferromagnetic end members are mechanically and electrically supported bysaid shield electrode.
 6. In combination with a magnetron ionizationgauge of the type having a longitudinally extending ion-collectorelectrode centrally positioned within a cylindrical anode and anelectron emitting cathode located offcenter between said ion-collectorand said anode and a shield electrode interposed along the axis of theanode electrode between the ion-collector electrode and theelectron-emitting electrode, the improvement comprising: a ferromagneticelectron-containment member adjacent one end of said anode electrode. 7.The combination recited in claim 1 wherein said ferromagneticelectron-containment member is mechanically and electrically secured tosaid shield electrode.
 8. The combination recited in claim 1 comprisinga second ferromagnetic electron-containment member adjacent the otherend of said anode electrode.
 9. The combination recited in claim 5wherein said second electron-containment member is mechanically andelectrically secured to said shield electrode.